1. Field of the Invention
The present invention relates to a process for treating exhaust gas containing dust, NOx, and SOx (hereinafter simply called xe2x80x9cexhaust gasxe2x80x9d) such as coal burned exhaust gas, heavy oil burned exhaust gas, and the like.
2. Summary of the Invention
At present, there are no methods for treating metal mercury or mercury compounds (hereinafter generically called xe2x80x9cmercuryxe2x80x9d unless otherwise noted) in exhaust gas and for performing denitration and desulfurization at the same time in the method for removing SO2 in a wet desulfurizing unit using an alkaline absorbing solution as an absorbent after removing NOx from the exhaust gas in a reduction denitrating unit.
There are known methods for removing mercury with a high concentration from exhaust gas using an adsorbent such as activated carbon or a selenium filter. These methods however require a special adsorption-removing unit and hence are not adaptable to treatment of vast exhaust gas such as flue gas.
There are also the methods in which exhaust gas and an absorbing solution are subjected to gas-liquid contact to remove mercury using a scrubber without alteration of a conventional exhaust gas treating system. In these methods, however, there is the problem that metal mercury which is less soluble in water cannot be almost removed though mercury compounds, e.g. HgCl2 which have relatively large solubility in water can be removed.
Combustion exhaust gas from a boiler 1 is introduced into a reduction denitrating unit 5. An ammonia injecting unit 2 for injecting, into the exhaust gas, NH3 supplied from an ammonia tank 3 is installed in the passage to the reduction denitrating unit 5. The exhaust gas into which NH3 is injected is denitrated in the reduction denitrating unit 5 in which NOx is converted into nitrogen by a selective reducing reaction shown by the following chemical formula:
4NO+4NH3+O2xe2x86x924N2+6H2O
NO+NO2+2NH3xe2x86x922N2+3H2O
The denitrated exhaust gas is allowed to flow through an air preheater 6 and a heat exchanger 7 to reach an electric precipitator 8 where dust is removed. Then SO2 in the exhaust gas is removed in a wet desulfurizing unit 9. In the case where mercury is contained as HgCl or HgCl2, HgCl or HgCl2 contained in the exhaust gas is dissolved in water through gas-liquid contact in the wet desulfurizing unit 9 whereby mercury is removed. However, in the case where mercury is contained as metal mercury which can be scarcely dissolved in water, mercury is removed at a lower removal rate and is almost vented from a stack 11 through a heat exchanger 10.
In order to remove metal mercury, it is required to install newly a mercury adsorption-type removing unit, e.g. an activated carbon filter, between the wet desulfurizing unit 9 and the heat exchanger 10 or between the heat exchanger 10 and the stack 11.
In the above method for treating mercury using the mercury adsorption-type removing unit, there is the problem of high facilities and operation costs in continuous treatment for vast exhaust gas having a lower mercury concentration. While, with intensified regulation of environmental pollutant emission, there is strong needs for developing inexpensive treating technologies for removing heavy metals such as mercury and the like.
It is an object of the present invention to provide a treating technology which is reduced in costs of facilities and operation.
The present inventors have studied various processes for removing mercury and as a result found that mercury can be efficiently removed in a wet desulfurizing unit by converting metal mercury into a water-soluble compound, to complete the present invention.
Accordingly, the above object can be attained by a provision of a process for treating exhaust gas comprising adding a mercury chlorinating agent such as ammonium chloride or HCl and ammonia to combustion exhaust gas containing NOx, SOx, and mercury to carry out reduction denitration in the presence of a solid catalyst; and wet-desulfurizing the denitrated exhaust gas using an alkaline absorbing solution.
In preferred embodiments of the present invention, the mercury chlorinating agent is ammonium chloride or HCl; and the solid catalyst includes a carrier composed of at least one compound selected from the group consisting of TiO2, SiO2, and ZrO2 and/or zeolite and at least one element selected from the group consisting of Pt, Ru, Rh, Pd, Ir, V, W, Mo, Ni, Co, Fe, Cr, Cu, and Mn is carried by said carrier.
Also, the object of the present invention can be attained by the provision of a combustion exhaust gas treating equipment comprising a reduction denitrating unit, a wet desulfurizing unit, a mercury chlorinating agent injecting unit, and an ammonia injecting unit; wherein a mercury chlorinating agent from said mercury chlorinating agent injecting unit and ammonia from said ammonia injecting unit are added to combustion exhaust gas containing NOx, SOx, and mercury to carry out reduction denitration in the presence of a solid catalyst; and the denitrated exhaust gas is wet-desulfurized using an alkaline absorbing solution.
In the present invention, it is possible to remove mercury in exhaust gas in a highly efficient manner using inexpensive chemicals such as HCl without a large change in a conventional system but with a simple reform, e.g. a HCl injecting unit attached. The present invention is therefore very advantageous in view of operating costs including plant costs, costs of chemicals, and maintenance costs.